1. Field of the Invention
The present invention relates to index table assemblies.
2. Description of the Related Art
Index table assemblies including a clamping device for bringing a rotary table into contact with a frame by moving the rotary table along a rotational axis thereof are known in the art (refer to, for example, Japanese Examined Utility Model Registration Application Publication No. 7-50133, FIG. 1 on page 4). In such an index table assembly, the rotary table is pressed against the frame over the entire circumference thereof when it is clamped, so that the rotary table is prevented from being tilted by an external force applied during workpiece processing. Accordingly, the workpiece-processing accuracy is maintained.
In an index table assembly shown in FIG. 9, a clamping device includes a plurality of hydraulic actuators arranged in a frame 1 along a circle centered on a rotational axis 15 at a constant interval. Each of the hydraulic actuators includes a piston 35 projecting from the frame 1, the piston 35 being fitted in a groove with a T-shaped cross section formed in a rotary table 2 such that a flange of the piston 35 engages with step in the groove. When the rotary table 2 is clamped, each of the pistons 35 receives hydraulic pressure and moves in a retracting direction. Accordingly, the rotary table 2 is pressed against the frame 1 by the flanges of the pistons 35 and comes into contact with the frame 1 over the entire circumference thereof. When the rotary table 2 is unclamped, the hydraulic pressure is removed and the rotary table 2 is released from the frame 1 so that the rotary table 2 can rotate.
As described above, when the rotary table 2 is unclamped, the hydraulic pressure is removed and the rotary table 2 is released from the frame 1 so that the rotary table 2 can rotate. However, since the rotary table 2 receives the load of onboard components such as a workpiece, a process table, and a jig, the rotary table 2 comes into contact with the frame 1 due to the load of the onboard components and its own load and large rotational resistance is caused. Therefore, a large amount of power is required to rotate the rotary table 2. In addition, a driving device of the rotary table 2, that is, a divider is quickly worn and the workpiece-processing accuracy is degraded. In particular, when, for example, the workpiece is unevenly placed and the center of gravity of the onboard components is distant from the rotational axis 15, the rotary table 2 is strongly pressed against the frame 1 in a local region of the frame 1 and the rotational resistance greatly increases. In addition, since the rotary table 2 does not always rotate by one turn, a clamping surface 2b of the rotary table 2 and a clamping surface 1b of the frame 1 are greatly worn in a certain region. In such a case, the perpendicularity of the clamping surfaces 2b and 1b of the rotary table 2 and the frame 1, respectively, relative to the rotational axis 15 is degraded. If the perpendicularity of the clamping surfaces 2b and 1b relative to the rotational axis 15 is degraded, the clamping surfaces 2b and 1b cannot be in even contact with each other over the entire circumference when the rotary table 2 is clamped and there is a risk that a gap will be generated in a certain region. In addition, even when the clamping surfaces 2b and 1b can be in contact with each other over the entire circumference, the perpendicularity of a workpiece-receiving surface of the rotary table 2 relative to the rotational axis 15 is degraded and the workpiece-processing accuracy is reduced accordingly. When a gap is generated between the clamping surfaces 2b and 1b, the clamping force is reduced and the rotary table 2 is tilted due to the external force applied during processing. Accordingly, also in this case, the workpiece-processing accuracy is reduced.
Therefore, an index table assembly disclosed in the above-mentioned publication includes elastically deformable rolling elements disposed between a rotating ring fixed to a rotary table and a frame. In this index table assembly, when the rotary table is clamped, the rotating ring is urged toward the frame, moves along the rotational axis while deforming the rolling elements, and is pressed against the frame. When the rotary table is unclamped, the urging force applied to the rotating ring is removed so that the rotary table can rotate, and the rolling elements return to their original shapes to support the rotating ring above the frame. Accordingly, the rotational resistance of the rotary table is equal to the rolling resistance of the rolling elements, and therefore the rotary table can easily rotate.
However, when the rotary table rotates, the rolling elements receive not only the load of the rotary table but also the load of onboard components such a workpiece, a process table, and a jig, and are therefore elastically deformed into a shape with an elliptical cross section. Accordingly, the rolling elements are constantly deformed into a shape with an elliptical cross section while they roll between the rotating ring and the frame. Since a large force is required for deforming the elastic elements, a large force is required for rolling the rolling elements. Therefore, the rotational resistance of the rotary table is also large in this index table assembly. In addition, since a large force is required for rolling the rolling elements while deforming them, sliding of the rolling elements relative to the rotating ring and the frame occurs. Accordingly, the rolling elements are unevenly worn with time and become harder to roll, which increases the degree of sliding of the rolling elements relative to the rotating ring and the frame. As a result, the rolling elements are quickly worn and the rotating ring comes into contact with the frame. Accordingly, the rotational resistance of the rotary table increases and the rotating ring and the frame are worn, which leads to a reduction in the workpiece-processing accuracy.